1. Field of the Invention
This invention relates generally to phased array antenna systems and more particularly to an orthogonal beam forming network for simultaneously providing multiple beams from a single aperture.
2. Description of the Prior Art
In radar systems, multiple beams allow parallel operation and a higher data rate than can be obtained from a single beam. In communications systems they allow for reuse of frequency and data transmission to different areas. Moreover, if these beams are comprised of sin x/x beams, they can be formed independently and losslessly. If there is provided an N element array with element spacing d, then N (sin x/x) beams can be formed which are spaced .lambda./N.times.d in space and where .lambda. is the operating wavelength. As is well known, the expression for the angle .alpha. of the progressive phase fronts required to form a set of orthogonal sin x/x beams can be stated as: ##EQU1## where m is the beam number and N is the number of elements in the array. Also as is well known, sin x/x beams have relatively high sidelobe levels but they can be combined to form beams with much lower sidelobe levels by providing an amplitude taper.
Until recently, the only network utilized for forming a lossless set of orthogonal sin x/x beams was the well known Butler matrix which has a binary number (2.sup.n) of input ports and output ports coupled to a binary number of antenna elements notwithstanding the fact that an article entitled, "Multiple Beams From Linear Arrays", J. P. Shelton and K. S. Kelleher, IRE Transactions On Antennas And Propagation, Mar., 1961, pp. 154-161, showed that other numbers of elements are theoretically possible using all values of 3.sup.m .times.2.sup.n through the use of a six port junction. One such six port network has furthermore been disclosed, in U.S. Pat. No. 4,231,040, entitled, "Simultaneous Multiple Beam Antenna Array Matrix And Method Thereof", S. Walker, Oct. 28, 1980. Such an arrangement, however, comprises a narrow bandwidth device since both the amplitude and output phase is far from being constant as a function of frequency.
Accordingly, it is an object of the present invention to provide an improvement in orthogonal beam forming networks coupled to a plurality of phased array antenna elements.
Another object of the invention is to provide an improvement in non-binary matrices for forming multiple antenna beams.
Still another object of the invention is to provide an improved six port junction for forming orthogonal antenna beams in a phased array.